A bus full of Kennedy assassination buffs touring Dallas is hit by a car, and several of the conspiratorialists are killed. Their souls are whisked straight to Heaven, where like all newcomers, they get a brief welcome from God himself.

After explaining where the bathrooms are and what time dinner is served, God throws the floor open to questions. "Ask me anything," he urges them. "In the Hereafter, we have no secrets."

One of the conspiracy buffs immediately asks: "Can you tell us who really killed President Kennedy?" God, nodding solemnly, replies: "Sure. It was Lee Harvey Oswald, acting alone."

Sadly, this little joke is no joke. I had planned to start this piece with a mocking claim that, while watching the PBS documentary Oswald's Ghost," I had pinpointed yet another suspect in the Kennedy assassination: journalist and historian Priscilla Johnson McMillan. McMillan, author of several books, including a biography of Oswald and his wife called "Marina and Lee," worked for Kennedy briefly in 1953 when he was the junior senator from Massachusetts. A few years later, she became a foreign correspondent and interviewed Oswald when he defected to the Soviet Union in 1959, making her probably the only person in the world who knew both men. Obviously she must have been a kingpin in the assassination plot, I was going to say ... until I Googled her name and found tens of thousands of conspiracy nuts have already reached the same conclusion, and unfortunately, they're not kidding.

So go ahead, put McMillan's name on the list, along with Mafia capos, renegade CIA officers, Corsican narcotraffickers, crazed Texas oilmen, vengeful South Vietnamese politicians, right-wing Cubans, left-wing Cubans, thrill-killing homosexuals, Lyndon Johnson, even Earl Warren, the Supreme Court chief justice who spent his days writing landmark expansions of American civil liberties and his nights, if Oliver Stone's film "JFK" is to be believed, plotting murders for the military-industrial complex.

If that last paragraph seems a little silly to you - a little paranoid - then you are a member of one of our country's most battered and dwindling minority groups, the Americans who believe that Oswald, acting alone and without help, shot President Kennedy in 1963. Fully 70 percent of the country believes there was a conspiracy, that Kennedy was stuck down not by a lone gunman with pretensions to grandeur but by vast, powerful forces pursuing secret agendas from the shadows.

"Oswald's Ghost," airing as an episode of the PBS documentary series "American Experience," is not going to change anybody's mind, and doesn't try to, at least not very hard. Though the sympathies of filmmaker Robert Stone (no relation to Oliver) obviously lie with the lone-gunman theory, he's mostly concerned with the paranoid and self-reinforcing ripples the assassination sent through American political culture.

As Stone has said in interviews, "Oswald's Ghost" is less a whodunnit than a what-the-whodunnit-done-to-us. With interviews with everybody from loopy conspiracy freaks like Mark Lane to erudite historians like Edward Jay Epstein and Robert Dallek, illustrated with an impressive collection of little-seen footage of Oswald and the assassination scene, it spins a tale of a society running off its tracks.

The bullets fired that morning in Dealey Plaza, "Oswald's Ghost" argues, ricocheted through history: Johnson, certain his predecessor had been killed by agents of Fidel Castro, tried to show he wasn't intimidated by dramatically (and disastrously) escalating the war in Vietnam. Youthful leftists, angered by the war and convinced of the futility of conventional politics by the assassinations of Kennedy, his brother Robert and Martin Luther King, retaliated with furious violence that ended in the rioting at the 1968 Democratic convention and guaranteed the election of Richard Nixon. That in turn touched off Watergate, which led to revelations of CIA druggings and murders - and, like some kind of endless loop of macabre tape, led back to the JFK assassination with the disclosure that the Kennedy brothers had been plotting the murder of Fidel Castro. Did Castro, as Johnson believed, strike back in Dallas?

The show's title is lifted from a section of Norman Mailer's biography "Oswald's Tale" in which Mailer ruefully observed that Oswald, a warped and frustrated loser while alive, fulfilled his ambition of rerouting history in death: "Can there be any American of our century who, having failed to gain stature while he was alive, now haunts us more?"

The irony is that Oswald achieved his mark on history only because nobody believes he was intelligent enough to have actually killed a president. The assassination's mythic hold over American politics derives from the idea that "somebody else" did it, that Oswald at most was a patsy for more sinister and powerful forces. If the picture painted by the Warren Commission was correct - that Kennedy was killed by a crackpot high school dropout whose wife wouldn't have sex with him the night before - the assassination would have been a tragedy, but little more. The nation would have mourned briefly, then moved on.

The real puzzle is that we consider it anything more than that. Modern forensics and the declassification of evidence from the FBI, CIA, the Warren Commission and a congressional investigation have left us with a mountain of evidence against Oswald: The bullets that struck Kennedy have been matched to Oswald's rifle, which was found at the scene of the crime bearing his palmprint. (The gun was purchased through the mail with an order form filled out in Oswald's handwriting and shipped to his post-office box. His wife took photos of him holding the rifle; she sent one to a friend, inscribed with a chuckle that would soon curdle in her throat: "Hunter of fascists, ha ha!") Bullets from the gun have also been linked to an earlier assassination attempt, against a right-wing Texas politician - a shooting Oswald confessed to his wife.

Autopsy photos make it clear that all the bullets that struck Kennedy and Texas Gov. John Connally were fired from the sixth floor of a Dallas building where Oswald worked, and from which he vanished within minutes of the crime. Five witnesses watched Oswald kill a Dallas cop who stopped him for questioning, and he was still carrying the murder weapon when he was arrested. If Los Angeles prosecutors had even half as much evidence against O.J. Simpson, he would be in jail today.

Of course, Oswald could have been the triggerman for a larger conspiracy. But why would anyone planning such a difficult and dangerous mission put it in the hands of a lifelong screw-up and misanthrope like Oswald? He quit the 10th grade to join the Marines, left the Marines to defect to the Soviet Union, left the Soviet Union for menial labor in the United States, and shortly before the assassination, was rejected in his attempt to re-defect to Moscow or Havana.

Mailer, once so perfervid in his belief that Kennedy was felled by a plot that he argued there were several different groups of separate conspirators shooting at the president that day in Dallas, finally admits in "Oswald's Ghost" that the evidence is overwhelming that Oswald acted alone.

"Like most conspiratorialists, I wanted it to be a conspiracy," he says in an interview taped last year before his death. "But I kept trying to think how a conspiracy could have put the thing together, and I must say, I failed notably ... The internal evidence just wasn't there."

But evidence has never had much to do with the way Americans see the Kennedy assassination. It's always been a national inkblot into which we project our fears and anxieties of the moment. The early conspiracy theories, at the height of the Cold War, tended to involve Soviet or Cuban agents. As American disillusionment with the Vietnam War grew, the CIA became the prime suspect.

During the gasoline shortages of the early 1970s, suspicion fell on the oil companies and later the Mafia. In the uncertain days after the Berlin Wall fell, Oliver Stone was wildly successful with a movie in which just about every public institution in America - Congress, the Pentagon, the Supreme Court, the FBI and CIA, the news media - participated in either the assassination or its coverup.

As "Oswald's Ghost" notes, that doesn't seem likely to change. "Americans are prone to paranoid thinking," says sociologist Todd Gitlin. "The belief that tiny cabals of people are actually pulling the strings runs back into the Book of Revelation."

Sadly, in a post-9/11 world where airliners turn into death ships that topple skyscrapers, where every traveler's shoe is a potential bomb and companies screen their mail for anthrax, there's plenty reason to be paranoid.